The present invention relates to metallization of aluminum nitride sintered body for use as an insulating material having a high thermal conductivity in an insulating substrate, heat sink, laser tube, and the like.
In recent years, the requirements for minimizing electronic instruments and improving their functions have increased prominently. As a result, semiconductors are rapidly developing towards improvements in integration density, multifunctionization, high speed, high output, and high responsibility, wherein more and more heat is generated by the semiconductor. Therefore, a base plate or substrate with higher heat-dissipation (or radiation) ability is needed instead of conventional Al.sub.2 O.sub.3 base plates or substrates. Various heat engines, industrial instruments, and the like are developing towards high output, too. Accordingly materials with higher heat-dissipation are generally required nowadays.
As the materials for substrates with high-dissipation ability, i.e., as the materials with high thermal conductivity, there are counted, for example, diamond, cubic BN (boron nitride), SiC (silicon carbide), BeO (beryllia), AlN (aluminum nitride), Si, and the like. However, these materials have following drawbacks. Diamond and cubic BN have a difficulty in manufacturing a product of such a size usable as a substrate and are very expensive. SiC cannot be used as an insulating material since SiC is semiconductive and inferior in electric characteristics such as electric insulating ability, dielectric constant, and the like. BeO, in spite of its excellent electric characteristics, has a difficulty in its stable supply since it is not manufactured in Japan thus depends on the import from foreign countries, due to the poisonous powder occurring in the compacting process, abrasive machining process, and the like. Si has inferior electric characteristics and a low mechanical strength, and hence has a limited use as a material for the substrate. Although AlN has advantages that it is excellent in electric characteristics, for example, high insulating ability, high dielectric brake down strength, low dielectric constant, etc. and that is can be treated with a normal sintering, it has a drawback that the metal layer cannot be formed on a required surface and that a bonding strength to metal is insufficient because AlN has a low wettability to metal. Thus employing AlN, therefore, multilayer substrates for high output, laser tubes, and the like have never been developed.